The community of corporations, government entities, and others that require secure communications has built a secure communications infrastructure in recent years. This infrastructure includes a large number of secure communication terminals that couple to the public switched communications network (PSTN) and other communication channels along with a structure for managing and distributing encryption keys. The secure terminals usually operate in both clear and secure modes.
In a typical scenario for establishing a secure call, one secure terminal initiates a call to another secure terminal. The call may, for example, be initiated by dialing a phone number. When the called party answers, both secure terminals first operate in their clear modes. In the clear modes no encryption or decryption of communications take place, and the secure terminals act like "plain old telephones" (POTs). However, if either party to the call wishes to conduct secure communications, that party may push a button on his or her secure terminal, causing that secure terminal to transmit a "initiate security setup" signal to the opposite terminal in the call. Thereafter, both secure terminals engage in a security setup process in which encryption keys, among other data, are exchanged. Upon completion of the security setup process, the secure terminals operate in their secure modes, and communications are encrypted for transmission and decrypted upon receipt.
In accordance with a particularly common type of secure terminal, conventional analog audio signals that are well known in the telephony industry are transmitted and received during the clear mode. However, the encryption and decryption operations are performed digitally. The secure mode utilizes vocoders, digital encryption units, modems, and like components to translate analog audio signals into digital signals, encrypt the digital signals, transmit the digital signals over a conventional PSTN, and perform complementary operations for received data. Thus, conventional analog telephone components are dedicated to clear communications while digitizing and encryption components are dedicated to secure communications. The dedication of various components to the different modes enhances the security provided for communications delivered in the secure mode.
Digital communication networks represent a modern trend. Such networks easily accommodate the secure mode of communication as implemented by the existing infrastructure of secure terminals. The digital signals may simply be communicated without signal translations otherwise performed by modems. However, such networks do not necessarily accommodate the clear mode of operation. While one might possibly design a type of secure terminal that also transmits clear communications digitally, this solution is undesirably costly and impractical. Such a solution could require an entire infrastructure of secure terminals that included additional digitizers, vocoders, decoders, and the like to permit clear digital communications while refraining from compromising security during the secure mode. Additional complication results from making such terminals compatible with conventional analog communications.